Automatic test equipment for checking printed circuit boards has long involved the use of "bed of nails" test fixtures on which the circuit board is mounted during testing. A typical test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact between measurement channels in the test equipment and designated test points on the circuit board under test, also referred to as the unit under test or "UUT". Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the arrangement of test probes for contacting test points on the board must be customized in a test fixture for that particular circuit board. Board design and fabrication data is used to determine what specific board features are to be tested by the fixture. A grid test fixture is typically fabricated by drilling patterns of holes in several rigid and nonconducting plates, assembling those plates with suitable fasteners and spacers to maintain said plates in a parallel, aligned position, and then mounting test pins or probes in the drilled holes. In a "determined" grid test fixture each plate has a hole pattern which is unique such that the test pin can only be inserted to provide an x, y and z translation between a unique feature on the UUT and a unique tester grid channel. In preparation for test, the circuit board is then positioned on the fixture precisely aligned with the array of test probes. During testing, the pins in the fixture are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred between the board and the tester through the fixture so that a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board can perform the actual test.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired" test fixture or a "dedicated" test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the channels of the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made be using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and high volume production boards where the higher cost of these fixtures can be amortized.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so-called "grid type" test fixture, also known as a "determined" fixture, in which the random pattern of test points on the board are contacted by rigid translator pins which transfer test signals to spring loaded interface pins arranged in a grid pattern in the tester. In these grid-type testers, fixturing is generally less complex, components can typically recovered for reuse with other circuit boards and can be produced at lower cost than wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly. It is the grid-type testers to which the present invention is directed.
A typical tester has thousands of switches and "channels." Each channel has several switches, and is addressable and serves as one coordinate in the "grid." The tester has spring loaded contacts which comprise the grid. The fixture contains rigid translator pins which conduct current from the grid channels to the UUT. In this way, the tester's computer can be made to test continuity and isolation in the UUT through the fixture. When testing a bare board on such a tester, a translator fixture supports and guides pins that conduct between a grid pattern of spring loaded probes in a grid base and an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins" are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture.
As mentioned, the fixture typically consists of several parallel translator plates with patterns of drilled holes for retaining a large number of pins extending through the translator plates. The holes drilled in the translator plates are typically drilled with diameters that are slightly oversized with respect to the diameter of the pins, so that the pins easily may be inserted into the holes in the various plates of the translator fixture. Thus, means must be provided for retaining the translator pins in the translator fixture, i.e. so as to prevent the pins from falling out of the fixture if the fixture is lifted up and/or turned upside down without supporting the bottoms of the pins.
There are several prior art approaches to retaining the translator pins in a translator fixture. One approach is described in U.S. Pat. No. 4,721,908. In pertinent part, the '908 patent describes a plurality of test pins disposed in an array plate in accordance with a grid, and a mask plate extending transversely to the test pins and having through bores through which the test pins extend. According to the '908 patent, means are provided for retaining the test pins in parallel alignment in the fixture with the test pins extending through respective through-bores in a mask plate, comprising a relatively stiff elastic plate formed e.g. of a reinforced foam material and mounted at a position spaced from said mask plate and extending parallel thereto. The test pins extend through the elastic plate in a manner such that the elastic material grasps the test pins, whereby the test pins are retained and maintained in parallel alignment due to the elasticity of said material. The '908 patent specification states, for example, at column 3, lines 25-47, that the elastic plate 4, which firmly engages the periphery of the test pins extending therethrough, prevents test pins 2 from dropping from the testing apparatus. The elastic plate described therein is designed to be sufficiently stable while exhibiting the required resistance so that it may be mounted on the apparatus in the manner of a stable plate. The elastic plate is described as comprising a plastic foam cushion such as open-cell polyurethane foam. The pins are pushed through the plastic foam and, in use, the foam cushion naturally applies a compressible lateral retaining force that holds the pins in place. This approach allows use of straight solid pins which have advantages of tighter spacing capabilities, low manufacturing cost, good probe deflection, and bi-directional use of the pins. However, disadvantages of this approach include the fact that the foam deteriorates over time, drag force from the plastic foam cushion reduces the compliancy of the translator pin force, and the foam cushion is unduly stiffened if translator pin density is high. The '908 patent also discloses, but does not claim, an alternative embodiment to the elastic plate, i.e. in which the pin retaining elastic plate comprises an elastic insert in the form of a thin, flexible sheet loosely placed in the space between top and bottom guide plates, both which are aligned with the predetermined basic array grid. (Column 4, lines 26-49).
Another prior art retention system consists of specially formed translator pins in which each pin has a single or multiple longitudinally spaced apart enlarged annular rings that project outwardly around the circumference of the pin. The pins are inserted in the translator fixture when the translator plates are assembled. When assembly of the fixture is completed, the rings on the pins are located inboard from the outer translator plates so that the rings can act as stops in preventing the pins from slipping out of the fixture if the fixture is either lifted up or turned upside down.
In another prior art fixture an enlarged ring at the bottom of the translator pin fits into openings within a two-part bottom plate that captures a lower portion of the pin within holes in the plates that act as stops on both sides of the ring. Both of these fixtures have the disadvantages that the probes are more expensive, probe deflection is reduced, there is added cost to the fixture, the pins are not bi-directional, pin loading time is greater, and the fixture requires disassembly for reconfiguring the pins or for serviceability.
In a more recent approach to the problem, a thin sheet of plastic film such as polyethylene terephthalate (sold under the trademark Mylar) is used as the pin retainer. The Mylar sheet is captured between two translator plates, and undersized holes are drilled in the Mylar sheet in alignment with the larger diameter holes in the translator plates. In addition, undercuts must be formed at specific position(s) in the translator pins, so that when the translator pins are inserted into the fixture, the undercuts are aligned with the undersized holes in the Mylar sheet. The enlarged sections of the pin on opposite sides of the undercut act as stops to prevent the probes from slipping out of the fixture. The undersized holes in the Mylar film act as a retainer and otherwise center the pin in the fixture. The advantage of this approach over the use of the foam cushion is that no lateral drag forces are produced by the Mylar film. However, a disadvantage is that it requires specially formed translator pins with undercuts, which greatly increases manufacturing costs when compared with straight solid pins, because of the requirement of grinding the undercuts in each pin.
The foregoing discussion of the prior art is taken in large part from U.S. Pat. No. 5,493,230, which describes a system for retaining translator pins in a translator fixture that is said to overcome the various disadvantages of the prior art systems described above. Specifically, the '230 patent teaches as a translator pin retention system, a thin rubber pin retention sheet. According to the '230 patent, the thin rubber pin retention sheet comprises a loosely mounted and highly flexible membrane redundant, i.e. formed from an elastomeric material, and has preformed pattern openings for the pins. The elastomeric sheet material is said to retain the pins in the fixture by applying a compressive force around the pins, while the flexibility of the thin sheet allows the pins to move with the retention sheet independently of the other pins and the pin supporting plates in the fixture, and essentially avoids drag forces or any restriction to compliant axial movement of the pins within the fixture.
A disadvantage common to the several prior art fixtures above, is the need to custom predrill the pin retention member, thus adding appreciably to fixture fabrication costs. The present invention provides a pin retention means which does not require custom predrilling. More particularly, the present invention employs a thin, flexible, plastic screen, having a multiplicity of interstitial openings in the mesh of the screen, which screen is located between fixture plates, which is freely movable and unmounted, as the pin retention member.